/Engine/Source/Runtime/Core/Public/Misc/ConfigCacheIni.h
Returns the Custom Config string, which if set will load additional config files from Config/Custom/{CustomConfig}/DefaultX.ini to allow different types of builds. It can be set from a game Target.cs file with CustomConfig = "Name". Or in development, it can be overridden with a -CustomConfig=Name command line parameter.
static const [FString](API\Runtime\Core\Containers\FString) & GetCustomConfigString()